Wind power has been in use by humans for thousands of years. Windmills have been used for irrigation, pumping, and milling grain since the 7th Century A.D. and wind has been propelling sailing ships since well before 3000 B.C. By the late 19th Century, the first electricity-producing, wind-powered machines were developed.
The modern wind-power industry began in the late 1970s. In modern installations, referred to as “wind farms,” multiple individual wind turbines are interconnected via a medium-voltage power-collection system and communications network. At a substation, this medium-voltage current is stepped up in voltage for connection to the high-voltage electric power grid.
The wind provides an estimated 72 terawatts of potentially economically extractable power. This is about five times the 2005 average global power consumption from all sources. In view of the significant environmental and/or geopolitical issues with most other major sources of energy (e.g., oil, coal, nuclear), there has been an increasing interest in the U.S. and abroad in satisfying at least a portion of energy demands via wind power. Although there are some environmental concerns about wind-energy production sites, notably aesthetic considerations (i.e., homeowners' “views”) and noise, they are largely mitigated by locating the installations at offshore locations.
The majority of potential high-quality offshore wind-energy production sites, including about 60 percent of the total U.S. offshore potential wind-energy sites, are in deepwater locations (i.e., greater than 60 meters depth). Relative to land-based or shallower offshore sites, these locations require relatively higher capital investment due to the costs of towing the shipyard-fabricated equipment to the offshore site, deploying it at site, and mooring it.
Floating platforms are most suitable for supporting wind turbines at deepwater sites. There are three types of floating platforms typically considered for this service: tension leg platforms, semi-submersible platforms, and spar platforms. FIG. 1 depicts these floating platforms, as well as two other fixed platforms for use in relatively shallower waters.
The tension-leg platform (TLP) is vertically moored via tethers or tendons grouped at each of the structure's corners. A group of tethers is called a “tension leg.” The tethers have relatively high axial stiffness (low elasticity), such that virtually all vertical motion of the platform is eliminated.
A semi-submersible platform obtains its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. With much of its hull structure submerged at a deep draft, the semi-submersible platform is minimally-affected by wave loadings. The operating deck can be located well above the sea level due to the stability of the concept (more advantageous for oil drilling applications than for wind turbines). Structural columns connect the pontoons and operating deck. Semi-submersible platforms can be ballasted up or down by altering the amount of flooding in buoyancy tanks. They are typically anchored to the seabed by combinations of chain, wire rope or polyester rope, although they can also be kept in place via dynamic positioning.
A spar platform consists of a large-diameter, single vertical cylinder supporting a deck. The name for the platform derives from the logs or “spars” that are moored vertically and used as buoys in shipping. The spar platform contains a deep-draft floating caisson, which is a hollow cylindrical structure similar to a very large buoy. Most of the structure is underwater and, as a consequence of its deep-draft hull, the spar platform has very favorable motion characteristics.
Spar platforms are moored to the seabed like tension leg platforms, but whereas a TLP has vertical tension tethers, a spar has more conventional mooring lines. Spars have to-date been designed in three configurations: the “conventional” one-piece cylindrical hull, the “truss spar” where the midsection is composed of truss elements connecting the upper buoyant hull (called a hard tank) with the bottom soft tank containing permanent ballast, and the “cell spar” which is built from multiple vertical cylinders stacked one above the other. The spar has more inherent stability than a TLP since it has a large counterweight at the bottom and does not depend on the mooring to hold it upright.
To date, only a spar platform has been used in a deepwater offshore wind turbine demonstration. To create and install the spar platform at that location required:                Fabricating the spar in the form of a long, cylindrical, steel hull at a shipyard, etc.        Towing the spar to a protected water site (calm waters);        Upending the spar using cranes located on barges, etc.        Adding a middle tower;        Adding an upper tower for supporting the rotor;        Fitting the rotor on the upper tower;        Towing the assemblage to the final location; and        Installing the anchoring system.        
As indicated above, current steel-spar technology for offshore wind-power installations relies on upending of the spar and assembly of the tower in protected deep waters. There are relatively few locations, such as Norwegian fjords, that meet these depth and weather-protection criteria. A different approach to the fabrication and installation of a deepwater offshore floating platform could greatly expand the number of offshore locations at which wind turbines can be installed.